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Abstract:
Introduction
For a successful interaction with our environment it is necessary to gather information about form and motion of
objects. Form and motion is, however, processed along specialized pathways. Here, we focused on the
interactions between form and motion processing in the context of an object apparent motion paradigm called
apparent rotation (1).
Methods
We presented in rapid succession a perspective view of a three-dimensional object and a rotated version of the
same object (Fig. 1). Herby we induced the illusion of a rigid 3D-object rotation (apparent rotation). It has been
proposed on grounds of priming experiments that the intermediate rotation positions are explicitly represented in
the visual system (2). To find regions involved in the explicit representation of the illusory rotation path we
designed a functional magnetic resonance imaging experiment using an adaptation design (fMRI-A) (3). In an
event-related fMRI-A experiment at 3Tesla (TR=1s, TE=40ms, FA=60°, voxel size= 3 x 3 x 5 mm3 ) we
presented first a pair of visual stimuli to induce the apparent rotation illusion (object and rotated object), followed
by one of five test stimuli consisting of the same object rotated in space. A test stimulus was either a repetition of
the second position (repeated, maximal adaptation), a stimulus rotated to an intermediate position (intermediate,
possibly adapted), two extrapolated positions (continuous and reverse extrapolation) not on the rotation path
(minimal or no adaptation), and an object that was rotated along an additional axis (novel, not adapted) (Fig. 1).
For a region that combines motion and object information we predict adaptation of activation for the presented
objects, and for those that are on the rotation path (predicted adaptation profile: repeated < intermediate <
continuous extrapolation < reverse extrapolation = novel). Subjects classified the test objects to be rotated
versions of the same or of a different object. We used BrainVoyager™ for stimulus generation and data analysis.
Results
We found two regions with an activation profile that indicated adaptation for the presented objects and for the
interpolated illusory objects (Fig. 2): one in the intraparietal sulcus (IPS +/-30, -68, 17) and another around the
anterior part of the insula (+/-32, 16, 10) in the frontal lobe. In both regions the BOLD-response to the
intermediate stimulus (red) and the repeated one (orange) was significantly lower than to the stimuli, which lay
outside the apparent motion path (continuous [blue] and reverse extrapolation [turquoise], novel [green]).
Psychophysical data: The continuous extrapolated stimulus is less often recognized as a rotated version of the
same object than the intermediate stimulus (Fig. 3), although both are rotated for the same amount. We can
conclude therefore that subjects recognized adapted objects more easily.
Conclusion
We conclude that IPS and INS play an important role in combining object and motion properties. In future
experiments we will try to replicate our results in the context of an additional centre task.
